US investigators are working to find out what caused a stretch limousine to erupt into flames as it crossed a bridge over the San Francisco Bay, killing a new bride and four of her friends.

Four other women suffered burns or smoke inhalation when the white Lincoln Town Car caught fire while travelling westbound over the San Mateo-Hayward bridge on its way to a party at a hotel in Foster City, located about 40km south of San Francisco overlooking the bay, the California Highway Patrol said.

The San Francisco Chronicle newspaper reported that among the dead was new bride Neriza Fojas, 31, who had been celebrating her recent marriage with eight female friends, all of them in their 30s or 40s.

Four of the women died alongside Fojas as they tried to escape from the burning limousine, law enforcement officials said. The four surviving women remained in hospital. The driver was uninjured.

The San Mateo County medical examiner, Robert Foucrault, told reporters at a press conference that the deceased women were found clustered around a small opening in a partition between the passenger compartment and driver's seat.

"When we got there ... they were up against the partition," Foucrault said. "They were getting away from the fire and that's why they were in the front towards the partition. You could also probably say that they were trying to get out as well."

Law enforcement officials said it was too early to determine what caused the 1999 Town Car to catch fire at its rear end, pending an inspection of the limousine by mechanical experts and fire department investigators.

California Highway Patrol Captain Mike Maskarich said there was no evidence that the limo had been involved in a collision or struck any debris in the road. He said that it was carrying one more passenger than the eight than had been authorised.

'THE GRIEF, THE ANGUISH'

"It was a horrible incident that happened," Kultar Singh, the owner of the car company, Limo Stop, told Reuters. "I send my condolences to the families that lost their lives. I'm very saddened."

The limousine's driver, Orville Brown, told CNN in an interview that he had picked up the nine women in Alameda, California, and that "everything was going fine" at first as he drove across the bridge.

At some point one of the women knocked on the partition and complained of smoke in the passenger compartment, but because of loud music playing in the car he at first misunderstood and thought the woman was asking to smoke a cigarette.

"I looked back and I just saw the grief, the anguish on her face and I started to smell the smoke, started seeing smoke, so I pulled over," Brown told CNN. "As soon as I pulled over, one of the ladies hops through the partition."

Brown said that two other cars pulled over and that their drivers tried to help, but by the time the rear door was opened it was already too late for the women who had been trapped inside the passenger compartment.

"It was horrific, just something that I never would have imagined to have happened," he said.

Brown told CNN that he was not aware of any mechanical issues with the limo and didn't know what caused the fire.

Foster City Fire Department investigator John Mapes told Reuters he had interviewed two of the surviving women, who told him that they had escaped the flames by climbing through the small window in the partition. Mapes said the limousine was still intact but badly damaged by fire.

The Chronicle reported that Fojas, a registered nurse who worked at a Fresno hospital, was heading to a bridal celebration at a hotel in Foster City where her husband was waiting. The couple was also planning to travel to her native Philippines next month for another ceremony with her family.

The five women died on the same day that a 26-year-old woman was killed after falling out of a bachelorette party bus onto a busy Kansas freeway and was struck by at least three vehicles.

Jamie N. Frecks was one of 17 women, including the bride-to-be, riding on the bus when she fell out of a side door.